1. Field of the Invention
The present invention relates to a semiconductor laser device to be used in minidisc players and the like, and a method for fabricating the same. More specifically, the present invention relates to a semiconductor laser device having high performance, low operating voltage, and long service life, and also to a method for fabricating the same.
2. Description of the Prior Art
A conventional semiconductor laser device 101 is constructed in the following way, as shown in FIG. 7. A semiconductor multilayer film 108 is stacked on a substrate 102 composed of, for example, n-type GaAs, the semiconductor multilayer film 108 being comprised of a buffer layer 103 composed of Se-doped n-type GaAs, a lower clad layer 104 composed of Se-doped n-type Al.sub.0.5 Ga.sub.0.5 As, an active layer 105 composed of Al.sub.0.13 Ga.sub.0.87 As, an upper clad layer 106 composed of Zn-doped p-type Al.sub.0.5 Ga.sub.0.5 As, and a current blocking layer 107 composed of Se-doped n-type AlGaAs. Further thereon stacked are a clad layer 109 composed of Zn-doped p-type Al.sub.0.5 Ga.sub.0.5 As and a contact layer 110 composed of Zn-doped p-type GaAs, one by one. On top and bottom surfaces of the semiconductor laser device, formed are a p (positive) electrode 111 and an n (negative) electrode 112 composed of, for example, Au--Ge and Au--Zn.
The method for fabricating the above semiconductor laser device is shown in FIGS. 8A, 8B, 8C, and 8D. First, as shown in FIG. 8A, a semiconductor multilayer film 108 is grown one layer after another by an MOCVD (Metal Organic Chemical Vapor Deposition) process on a wafer substrate 102a composed of, for example, n-type GaAs, the semiconductor multilayer film 108 comprising a buffer layer (thickness 2 .mu.m) 103 composed of Se-doped n-type GaAs, a lower clad layer (thickness 1 .mu.m) 104 composed of Se-doped n-type Al.sub.0.5 Ga.sub.0.5 As, an active layer (thickness 0.08 .mu.m) 105 composed of Al.sub.0.13 Ga.sub.0.87 As, an upper clad layer (thickness 0.3 .mu.m) 106 composed of Zn-doped p-type Al.sub.0.5 Ga.sub.0.5 As, and a current blocking layer (thickness 1 .mu.m) 107 composed of Se-doped n-type AlGaAs. Thereafter, as shown in FIG. 8B, a striped groove (top width approx. 4 .mu.m) 113 is formed by etching so as to be bored through the current blocking layer 107.
Further thereon, as shown in FIG. 8C, a clad layer (thickness inside the striped groove 2 .mu.m, thickness outside 1 .mu.m) 109 composed of Zn-doped p-type Al.sub.0.5 Ga.sub.0.5 As and a contact layer (thickness 45 .mu.m) 110 composed of Zn-doped p-type GaAs are grown one by one by a liquid phase epitaxial growth process (hereinafter, referred to as LPE growth process). Finally, a p (positive) electrode 111 and an n (negative) electrode 112 composed of, for example, Au-Ge and Au-Zn are formed and thereafter the wafer is divided into individual semiconductor laser devices.
In the above method of fabricating a semiconductor laser device, the process using the LPE growth process is carried out by a growth slide board 115 made of carbon as shown in FIGS. 9A through 9D. As shown in FIG. 9A, a substrate 102b on which the semiconductor multilayer film has been grown is set at a recess of a base 116, while an LPE-grown clad-layer growth use fused liquid 117 and a contact-layer growth use fused liquid 118 are set at growth fused liquid sumps. The liquid phase epitaxial growth (hereinafter, referred to as LPE growth) is carried out while the temperature is gradually lowered. As shown in FIGS. 9B and 9C, a slider 120 is pulled by a slide bar 119 at a time point when a specified time or temperature is reached, so that the growth fused liquids 117, 118 are switched over. Finally, as shown in FIG. 9D, the slider 120 is further pulled so that the growth fused liquid 118 is removed from the substrate 102b. Thus, the growth is completed.
The above conventional semiconductor light-emitting device has had the following problems:
(1) A semiconductor laser device like the prior art example uses Zn as the p-type impurity for the LPE-grown clad layer and the contact layer. However, Zn has an high vapor pressure and therefore easy to evaporate such that even if it is charged in the growth fused liquid, its concentration in the fused liquid will not be stabilized.
(2) The aforementioned p-type impurity Zn is intensely diffused in the solid phase, being diffused into the lower semiconductor multilayer film during the growth at high temperature, affecting the carrier concentration. This causes deviations from the design structure of the semiconductor laser device and moreover leads to reduction in reproducibility.
(3) The contact layer of the semiconductor laser device such as shown in the prior art example is grown to a large thickness as much as 45 .mu.m, requiring a long time to attain such a growth. However, the growth temperature decreases with time so that the p-type impurity Zn is captured into the growing layer in gradually decreasing amounts. Therefore, the resistance of the contact layer becomes higher as the growth progresses, so that the operating voltage of the semiconductor laser device becomes higher.
(4) The slider is pulled at an end of the growth of the contact layer so that the contact layer growth use fused liquid is removed from the substrate on which the semiconductor multilayer film has been grown, whereby the growth is terminated. This method may cause the bottom portion of the slider to be damaged by an edge growth portion of the contact layer that forms an extremely thin, high grown portion around the substrate. This in turn may cause growth defects to occur in the next and following growths.
(5) Generally in semiconductor laser devices, the active layer for use of laser beam oscillation is advantageously arranged in a generally center of an end surface from which the laser beam is emitted, in terms of improvement in characteristics, relaxation of stress applied to the active layer, improvement in the operating life, and the like. In the semiconductor laser device and its fabricating method such as shown in the prior art example, however, an attempt to arrange the active layer generally in the center would require the contact layer to be grown to a thickness as much as 45 .mu.m, so that the p-type impurity Zn would be diffused into the lower semiconductor multilayer film during the growth. This may cause the semiconductor laser device to be deteriorated in characteristics, as still another problem.